Long term trends and future projections of rainfall in Southern Australia

12 December 2018

Information on long-term trends and future projections of rainfall in southern Australia has important implications for many sectors including water management, agriculture, transport and emergency management. This information can be used to raise awareness and help identify and minimise exposure to climate-related risk, and therefore to make climate-smart decisions for the future.

Hub researchers are building on Australia’s national climate change projections released in 2015 in an effort to continue developing our scientific understanding of the processes driving southern Australia’s rainfall. As a result we now know more about the causes of our declining winter rainfall, the changing seasonality of rainfall, and the occurrence of extreme rainfall events in southern Australia.

Current rainfall trends

While rainfall in southern Australian can vary greatly from year to year, in recent decades there has been a generally drier climate than the long-term average, particularly in winter. This trend is generally tracking at the drier end of the range of existing climate projections in key agricultural regions.

In south-west Western Australia, the trend to drier conditions has been accompanied by large reductions in inflows into the main water storage systems, and reducing groundwater levels in some catchments. Similarly, in south-eastern Australia many catchments have experienced a 50% decline in streamflow in recent years, which has had serious implications for urban water supply, environmental flows and agricultural/horticultural production. While this recent period included the Millennium Drought, there is some evidence that climate change has played a role in the decline in rainfall more generally in recent decades across southern Australia.

There are a number of major rain-bearing weather systems which impact rainfall in southern Australia, including cold fronts, cut-off lows, (including east coast lows) and thunderstorms. These weather systems are influenced by large scale atmospheric modes of variability, such as the Southern Annular Mode (SAM), which are in turn influenced by global warming. For example, SAM is projected to shift towards its more positive phase in a warmer climate resulting in less favourable conditions for winter rainfall across the mainland.

More drying to come…

Rainfall projections for the end of the century (2080–2099) show that as the concentration of global atmospheric greenhouse gases increases, we can expect a further decline in winter rainfall across much of southern Australia. This is particularly evident for south-western Western Australia. An exception is Tasmania, where there is medium model agreement of an increase in winter rainfall (of up to 20%) in association with projected increases in the strength of the westerlies.

Projected rainfall changes for summer and autumn are less clear due to the complex interplay between tropical and mid-latitude rain-bearing processes at that time of year. The exception is in Western Tasmania, with most climate models projecting a decrease in summer rainfall.

Near-term projections (2030s, 2050s) show less pronounced rainfall change and less difference between high and low emissions scenarios than later in the century. Observed winter rainfall drying trends are tracking at or below the dry end of the winter climate model projections for many regions of southern Australia.

…but flooding is still possible

While southern Australia is expected to get less total rainfall in general in the future, short-duration extreme rainfall (e.g. relatively high rainfall occurring over a few hours rather than multiple days) is projected to intensify, even in regions where mean rainfall decreases. Short-duration extremes can have major impacts, such as flash flooding – particularly in urban environments – which holds significant implications for infrastructure design, construction and maintenance into the future.

Climate change science to improve rainfall projections

The Earth Systems and Climate Change Hub’s research is improving our confidence in climate change projections and is increasing our scientific understanding of how the climate system works now and into the future. The Hub is also developing methods to deliver new projections of future water availability and hydrologic variables and metrics important to the water, agriculture and other key sectors. The Hub is also working to make science-based climate change data and information more user-friendly, so it can more easily be applied to inform risk assessment, policy development and adaptation planning activities.